1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically, to a silicon nitride film used in a capacitor formed in a semiconductor integrated circuit.
2. Description of Related Art
In a conventional semiconductor integrated circuit in accordance with a high voltage bipolar process, an epitaxial layer grown on a whole surface of a substrate is divided into a plurality of islands isolated from one another by PN junctions. In each of the islands thus formed, a circuit element such as a transistor, a capacitor or a resistor is formed.
For example, a capacitor is formed in some of the islands. The capacitor includes a lower electrode constituted of an diffusion layer formed in the epitaxial layer within the island, and a dielectric layer composed of a thin silicon oxide film and a thin silicon nitride film, which are formed after selectively removing in a first contact formation process an interlayer insulating film formed to cover the whole surface of the epitaxial layer. After formation of the thin silicon oxide film and the thin silicon nitride film, a metal layer such as an aluminum layer is sputtered so as to form not only an upper electrode of the capacitor but also a power supply line conductor in other regions.
Here, the silicon nitride used as the dielectric layer reforms or rectifies a curvature of the semiconductor substrate caused by a difference in thermal expansion coefficient between the semiconductor substrate and a silicon oxide film which constitutes the interlayer insulating film. H. Mikoshiba et al reported that a h.sub.FE parameter--current characteristics of transistors can be improved in a wide range by correcting the curvature of the semiconductor substrate (See Journal of Electrochemical Society, Vol. 123, No. 10, October 1976, pp. 1539.about.1545).
However, the silicon nitride film is formed not only in a region of the capacitor, but also on the whole of the semiconductor integrated circuit, and the silicon nitride film is selectively etched in only positions where the diffusion layer and a metal conductor are connected to each other.
In the conventional semiconductor integrated circuit, if a portion of the interlayer film other than predetermined opening formation positions is removed in the first contact formation process by any unexpected cause, a structure of a thin nitride film and a thin oxide film (called a "pin hole structure" hereinafter) is formed.
If the pin hole structure exists under the power supply line conductor, since the silicon nitride has a some degree of withstand voltage, integrated circuits having the pin hole structure will pass a product inspection conducted in the process of manufacturing the semiconductor integrated circuits, and therefore, delivered into a market.
However, even if the integrated circuit having the pin hole structure operates as a non-defective product, carriers are accumulated by injection of a current into the nitride film/oxide film from the power supply line conductor, so that a local electric field gradually becomes intense. If the gradually intensified local electric field exceeds a certain limit, the film is broken down.
In the case of a high voltage bipolar transistor, a voltage of about 50 V is applied between the power supply line and the semiconductor substrate. Therefore, failure of the semiconductor integrated circuit caused by the above mentioned breakdown will cause destruction of a package or failure of an external device. Thus, a shipping of the integrated circuits having the pin hole structure under the power supply line conductor has become a significant problem.